In real life, I have some family members who I classify as mean. But you know what, I still like them. And just because they can be mean to me sometimes doesn't mean that how I feel about them takes a hit. In fact, they are meaner to people they don't like than those they do.
So how is it realistic that my mean sim is happy but you know what, they'll just yell at their wife just because they feel like it.
Edit: I'm not sure how they would fix it, but my main problem is if I have two sims in a household and one of them has the mean trait, leaving them alone is a no-no. If I don't pay attention for a few minutes, the mean sim has autonomously queued up mean interactions with the other sim, even if they have a full relationship bar. It's worse when the sim is their best friend or soulmate. I mean, I would think with that status, the other sim would learn that the other person is just mean natured.
For me, people who are mean in general, its just who they are. I can't take it personally when they do it to every person they interact with. And if they are being mean to me in a way that is personal, well, they know exactly what they're doing. But usually it's because they are feeling tense, angry, or I did something that angered them and they want to get back at me. Or, they don't like me.